This invention relates to a tundish and is particularly concerned to provide a means of improving flow of molten metal through a tundish.
In the continuous casting of steel. molten steel is poured from a ladle into an intermediate vessel, a tundish, and from the tundish into one or more continuous casting moulds. For example, the tundish may feed two casting moulds. i.e. it is a two-strand tundish.
It is well known that unwanted non-metallic inclusions can be entrained in the steel in the tundish and a variety of means have been proposed to improve the steel quality by removing such inclusions before the steel passes from the tundish. Such means include the use of a layer of xe2x80x98activexe2x80x99 flux on the surface of the molten steel in the tundish. which flux captures unwanted inclusions. Also, such means include a variety of baffles, dams and weirs positioned in the tundish. In this context, a baffle is an obstruction to steel flow extending from the floor of the tundish to above the uppermost level of the molten steel, a dam is an obstruction protruding upwardly from the floor of the tundish and over which the steel must flow and a weir is an obstruction protruding downwardly into the molten steel and under which the steel must flow.
Such baffles, dams and weirs may be made of, or include a surface layer of an active material, e.g. alumina, to capture inclusions from the steel. They may have holes to allow passage of a portion of the steel through and they may be dimensioned and positioned to urge upward flow of the steel into a better contact with a surface layer of flux material.
Furthermore, it is also known to position an erosion-resistant impact pad on the floor of the tundish to receive the incoming stream of molten steel from a ladle.
Baffles, damds, weirs and impact pads will herein be referred to collectively as tundish furniture.
Impact pads have also been designed to minimise surface turbulence in the tundish in addition to their primary erosion-resistance role. For example, in U.S. Pat. No. 5,169,591, an impact pad is disclosed which has a base to receive an incoming ladle stream and a sidewall extending upwardly along the periphery of the base. The upwardly extending sidewall includes an inner surface having an undercut portion facing the incoming ladle stream, the undercut portion extending along the length of the inner surface and having a surface shaped to receive and reverse the direction of fluid flow generated by the incoming ladle stream. Molten metal flowing from the impact pad can pass through holes in a baffle located between the impact pad and the outlet from the tundish.
By reducing pour zone turbulence, the risk of the surface covering flux layer being broken and exposing the steel to air can be reduced, thereby reducing levels of steel reoxidation and heat loss.
The present invention aims to provide an improved arrangement of tundish furniture for a tundish that reduces risk of pour zone turbulence and improves flow patterns for steady state casting in the tundish so that inclusion removal can also be improved tundish so that inclusion removal can also be improved and the amount of slag/flux cover layer break up and entrainment at ladle changes can be reduced.
Accordingly, in one aspect the invention provides a tundish having an outlet in its base the outlet being spaced longitudinally of the tundish from a pour zone, the pour zone being positioned to receive a stream of molten steel from a ladle, an impact pad on the floor of the tundish in the pour zone, the impact pad comprising a base having an, impact surface, an upwardly extending sidewall along the periphery of the base, the sidewall having an inner surface having an undercut portion to face the incoming steel stream and the undercut portion having a surface shaped to receive and reverse the direction of flow of the incoming stream, and a dam positioned between the impact pad and the outlet, the dam having one or more holes to allow through passage of a proportion of the steel whereby a proportion of the steel can pass through the dam and a proportion of the steel can pass over the dam characterised in that the dam extends upwardly from the tundish floor for from about 40% to 60% of the height of the normal maximum level of steel in the tundish.
In another aspect the invention provides a kit of parts in the form of furniture for a tundish, the kit comprising an impact pad and a dam as defined in the immediately preceding paragraph.
Preferably, the tundish has two outlets longitudinally spaced one on each side of the pour zone and a dam is provided between the pour zone and each outlet.
Preferably, each dam has a pair of holes uniformly spaced across its width and the holes are preferably positioned above the tundish floor with the distance from the floor to the closest edge of the hole being from 25 mm to 50% of the height of the dam. The holes may be of circular cross-section, i.e. the passageways through the dam are cylindrical, although this is not essential, and they may be, for example, of elliptical or other shape.
The holes may extend horizontally through the dam but, in a preferred embodiment, they are angled downwardly, e.g. at an angle of from 30xc2x0 to 60xc2x0 to the horizontal from the pour zone side to the outlet side of the dam. In this instance, the heights of the hole centres referred to above are measured on the upstream, i.e. impact pad side, of the dam.
The holes may be, for example, of 5 to 15 cm in diameter for a dam across the full width of tundish, the dam being of height 40 cm and the tundish having a steel working level of 80 cm.
The impact pad may, for example, be of the type disclosed in the aforementioned U.S. Pat. No. 5,169,591. Alternatively, it may be a modified form of impact pad as described in our co-pending U.S. patent application Ser. No. 08/338,123, filed Nov. 9, 1994. In that application is described an impact pad having a base and an endless outer sidewall extending upwardly from the base and enclosing an interior space having an upper opening for receiving a stream of molten metal, the outer sidewall including an annular inner surface having at least a first portion extending inwardly and upwardly towards the opening and preferably a second portion extending outwardly and upwardly towards the first portion, whereby a downwardly directed stream of molten metal striking the impact surface of the base is directed outwardly towards the annular inner surface and is then redirected upwardly and inwardly towards the incoming molten metal stream. Also, there is described an impact pad having a base and an endless sidewall extending upwardly from the base, a top surface substantially parallel to the impact surface of the base and connected to the sidewall and defining a non-uniform opening therein, the non-uniform opening having a long dimension and a short dimension perpendicular to the long dimension, the sidewall having an interior face shaped so that molten metal contacting the impact surface flows outwardly then is turned inwardly and directed upwardly by the interior face of the sidewall and then flows out of the opening. In this latter embodiment the opening is intended to be aligned with the long dimension of the tundish.
The optimum positioning and size of the dams relative to the outlets and the pour region will, of course, vary from tundish to tundish but will be readily determinable by the average skilled man of the art who, for example, frequently uses mathematical means and/or water-modelling as a means of determining tundish furniture design and positioning.
The invention has been found to provide considerable flow improvements in a tundish. The flow benefits include:
i) increased minimum residence time
ii) decreased dead volume
iii) increased median residence time
iv) increased surface directed flow.
These characteristics all aid improved inclusion removal. Clearly, increased residence time increases likelihood of inclusion removal from the steel. Decreased dead volume reduces those regions of the tundish where flow becomes static thereby reducing temperature homogenisation and making inclusion removal less likely. Increased surface directed flow improves contact between the steel and an active flux covering layer thereby promoting collection of inclusions.
v) Reduced steel re-oxidation is also achieved by reduced turbulence, particularly on pouring steel from a ladle into an empty tundish and during ladle changes during sequence casting. Thus, the use of the particular type of impact pad dampens the energy of the steel poured into the tundish and the reduced turbulence so achieved reduces the exposure of steel to air thereby reducing the formation of oxide inclusions, e.g. alumina and iron oxides, commonly produced during teeming of molten steel.
vi) A reduction is achieved in ladle slag exiting from the tundish outlets.
vii) A reduction is achieved in transitional mixed steel grades (i.e. steels of different chemical constitution), during sequence casting thereby improving yield.
viii) A surface layer of active tundish flux can be used over the entire tundish thereby improving inclusion pickup.
Moreover, the invention enables a reduction to be achieved in the tundish furniture material required compared to conventional pour pad/baffle systems or pour pad/dam/weir systems.